Heap leaching

process used for the recovery of copper and gold from weathered ore and material from mine dumps; material is laid to a thickness of 20 feet (6 meters) in alternately fine and coarse beds and treated with water at intervals
during which oxidation occurs; liquor that runs off is treated with scrap iron to precipitate coppe

In the heap leach process, ore is crushed to the size of gravel. The crushed ore is laid out in heaps on top of a liner. The liners are typically made of an impermeable geotextile that can withstand degradation from light, water, chemicals and other forces. The liner material is placed on top of clay or other natural product that is relatively impermeable. Cyanide solution is dribbled over the heap by an irrigation system of pipes and hoses. As the solution trickles through the heap, it reacts with tiny gold grains, which preferentially go into solution. The gold-bearing solution is decanted from the bottom of the heap liner, and gold is then recovered from the solution.Photo 1: Heap leach. This heap has had all the gold extracted. Cyanide has been destroyed, and the surface is being revegetated. The irrigation system used to introduce the cyanide solution will be used to provide water for the new vegetation.With the vat leach process, the ore is crushed, then ground to a powder. The ground ore is placed in a large vat where it is agitated in a cyanide solution. As with heap leaching, the gold is preferentially dissolved by the cyanide solution. Once the reaction is complete, the gold-bearing solution is drawn off, and gold is recovered from the solution.Photo 2: Vat leach. Crushed and ground gold ore is agitated in a cyanide solution.Whichever, method is used, heap leaching or vat leaching, the next step is to recover the gold from the cyanide solution. The most common method utilizes activated carbon. The carbon is introduced into the solution, and gold adsorbs to the surface of the carbon particles. The loaded carbon particles are screened out of the solution. A small amount of hot, caustic cyanide solution is then used to release the gold from the carbon surface. The resulting solution, rich in gold, is subjected to an electrolytic process. An electric circuit set up in large containers of the solution causes gold to plate out. The plates of gold are further refined by smelting. In this process, the gold plates are melted, and poured into ingots, where impurities are separated. The resulting bars or ingots contain 70% to 90% gold, with the remaining ******* being silver. Further refining by secondary smelting or electro-refining is then done to produce almost perfectly pure gold.